Lithography is a process for producing a pattern on a semiconductor wafer. The pattern is produced by first exposing a pattern etched into a mask onto a semiconductor wafer coated with a resist material. The projected image of the pattern changes the composition of the resist material on the semiconductor wafer which is then removed to leave a matching pattern on the semiconductor wafer for further processing.
Depending upon the particular lithography application, the mask needs to satisfy several different requirements. The challenge is in finding a material or materials for use as the mask which will satisfy these requirements.
Historically, optical masking has been performed using a layer of chromium rich chromium nitride over fused silica, overcoated with a chromium oxinitride anti-reflective (“AR”) layer to reduce reflectivity below 10%. Chromium (“Cr”) is an excellent choice from optical the thermomechanical standpoints at wavelengths where it is sufficiently opaque, i.e. wavelengths generally above 180 nm. The spectra of chromium metal shown in FIG. 1 illustrates why it has been such a good mask from about 193 nm to about 436 nm. Referring to FIG. 2, the optical density (plotted here at log (transmission)) of a 1000 Å Cr metal film is shown. Again for wavelengths between about 193 nm and 500 nm, chromium is an excellent choice. The optical density of Cr film is near 5.0 in the ultraviolet (“UV”) range and it decreased to ˜4.5 at 193 nm. As thinner Cr metal films are used in manufacturing (800 Å for instance), the optical density above 190 nm may still be sufficiently high (above 4.0).
However, as shown in FIG. 1, chromium metal is a less desirable choice as a mask for wavelengths below 193 nm. Additionally, as shown in FIG. 2, at 157 nm, the optical density of the Cr metal film is 3.5 for a 1000 Å film and as low as 3.0 for a 800 Å film. At these optical densities, mask modulation is likely to be low for imaging of fine feature geometry. Unfortunately, a masking film thickness of over about 800 Å is undesirable for 157 nm applications because of the aspect ratio requirements of features smaller than 300 nm. Accordingly, it appears that chromium mask materials may not be adequate below 193 nm, in particular at potential vacuum ultraviolet (“VUV”) wavelengths, i.e. from about 120 nm to 180 nm, such as 157 nm.
Another factor in selecting a mask for use in optical lithography below 180 nm is the etch characteristics of the mask. The mask must have suitable etch characteristics with selectivity to the underlying substrate and to the resist material. In other words, the film on the substrate which forms the mask must be made of a material or materials which can be etched to form the pattern to be replicated on semiconductor wafers without significant loss to the underlying substrate or to the resist material.
As yet, an appropriate replacement material or materials for chromium films for use as the mask for lithography at or below 180 nm, i.e. a mask with the desired optical properties and etch characteristics, has not been found.